Background and Business Description Rehabilitation Exit Support Agreement (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Feb. 12, 2018
Cancellation of Surplus Notes and Accrued Interest			$ 552,320,000
Cancellation of Surplus Notes Accrued Interest			257,200,000
SatisfactionAndDischargeOfSegregatedAccountDeferredAmounts			3,856,992,000
Gain Contingency, Unrecorded Amount	$ 1,400,000,000
Market Value of Secured Note Collateral	$ 367,816,000
Tier 2 Notes			240,000,000
Tier 2 Note Security in RMBS Litigation Settlement Proceeds			1,600,000,000
One Time Interest Payment on Surplus Notes			13,501,000
Deferred Payment Obligation [Member]
Discount Amount			0.065
Surplus Notes [Member]
Cash Payment Amount			0.40
Payment Amount in New Secured Notes			0.41
Payment Amount in Existing Surplus Notes			0.125
Ambac Assurance [Member]
Secured Notes Received by Ambac LSNI and Ambac Assurance			643,583,000
Ambac [Member] | Deferred Payment Obligation [Member]
One Dollar Outstanding			1.00
Payment Amount in New Secured Notes			0.91
Discount Amount			0.09
AFG [Member]
Secured Notes Received by Ambac LSNI and Ambac Assurance			124,881,000
Ambac [Member]
One Time Interest Payment on Surplus Notes			$ 2,618,000
5.1% Surplus Notes, General Account, Due 2020 [Member] | Ambac Assurance Corporation [Member] | Successor [Member]
Debt Instrument, Interest Rate, Stated Percentage	5.10%	5.10%
Debt Instruments Maturity Year	2020	2020